As dimensions decrease in electronics technology, among the major issues that arise are limits to power that can be extracted as dissipation occurs in smaller regions and the increasing variability in devices which depend on collective effects (the √n effect). Decreasing size and increasing density permits larger information capability in processing and in storage. Non-volatile memories are ubiquitous, in use in digital cameras, cell phones, music players, computers, and everywhere else where non-volatile retention with rapid reading is of interest. Non-volatile memories, if integrated with electronics and allowing fast and low power data and program transfer, would permit new methods of information processing that may require less power. Achieving lower power in digital processing is also of interest so that more information processing capability can be integrated in smaller regions.
Semiconductor non-volatile memories provide speeds which while slower than of SRAMs, are faster than possible through other means, e.g., magnetic disks. The most common forms of non-volatile memory are various manifestations of electrically erasable and programmable memory structures employing a floating gate region in which charge is stored. Many new manifestations of this structure use few electrons, single electron, and defects to lower the power and to allow scaling of dimensions to dimensions lower than those possible with continuous floating gate regions. Memories have also employed defects and storage on the back of a silicon channel, thus allowing simultaneous transistor and memory capabilities. In these structures, charge is injected into the storage region and its storage there changes the state of the device. The energy of the electrons can cause defects and limits the useful number of cycles that the memory can be applied in addition to the power dissipation of the process.
Digital logic relies on field-effect, i.e., transport of electrons between two electrodes (source and drain) under the influence of a field where the charge is also simultaneously controlled by application of a field through a third electrode (gate). The turn-on of this device from off-state (where a barrier is present between source and drain) to on-state (where conduction is field modulated) occurs by control of the barrier between source and drain during the quasi-off state. Higher energy electrons (those in the tail in energy) can overcome the barrier, so the turn-on process has an exponential dependence that is at best 60 mV/decade of change in current. This limits the lowest voltages that can be applied to the device before a collection of them malfunctions.
Charge transport, e.g., electron transport and rapid electromagnetic field changes lead to power dissipation. Electron transport into the floating gates and out lead to high energy electron mediated defect generation. Thus, both non-volatile memories and transistors, where electron transport is involved, whether in the channel region between source and drain, or in between gate and the channel, have limits in voltages, power, dimensions, and the number of times the states can be changed.
These phenomena limit the usefulness at small dimensions because of the use of collective effects related to the charge densities in the two-dimensional regions (the channel) that are decreasing in size and the number of carriers that one works with which in these cases are limited to some density of states available multiplied by volume or area.
Field-effect's major attribute is that until reaching its limit, the turn-on behavior of the device is immune to the length scale. Transistor's threshold voltage does not change with gate length until it reaches the limit of smaller dimension and variability become important.
Use of phenomena either independent or coupled to field-effect that can overcome the collective effect limit arising from charge or dopant statistics may allow further reduction in power and increase in usefulness of electronic devices.
One such phenomenon is phase transition, a collective effect in which an ensemble of atoms change in properties under the influence of field, temperature, etc. Ferroelectric, ferromagnetic, metal-insulator, . . . , are examples of such transitions. CDROMs and DVDs employ phase transition materials whose optical reflectance is changed. Ferroelectric memories employ phase transition materials whose polarity (electric polarization) is reversed. Ferromagnetic storage, such as in disk drives, employ polarity (magnetic polarization) for bit information storage.